The present invention relates to an image size transformation method for an orthogonal transformation coded image, particularly to that which enlarges and reduces the image size of coded data in respect with compression coding of an image using an orthogonal transformation.
A conventional image size transformation method for an orthogonal transformation coded image is defined as a method for acquiring coded data of an enlarged or a reduced image size from coded data of an image compressed by an orthogonal transformation.
As to the image size transformation method for obtaining the coded data, the following image size transformation method is the most typical. The coded data, i.e. orthogonal transformation coefficients, is once reproduced into an image by an inversion. Then the image size is enlarged or reduced by interpolating or thinning out pixels at the image space, after which the image goes through compression coding by the application of the orthogonal transformation for the second time. FIG. 1 shows a processing procedure of such enlargement process. Note that a reduction process would require the same processing procedure. In this conventional image size transformation method, however, not only enlargement/reduction process of the image size is required but also an inversion of the orthogonal transformation coefficients and a repetition of the. orthogonal transformation process are required. Accordingly, it is noted as a disadvantage that the whole image size transformation process would become complicated.
This sort of image size enlargement method is applied to a spatial scalable coding system in connection with the IS 13818-2 standard which is being internationally standardized by ISO/IEC JTCI/SC29/WG11. In this system, an image is coded with a low resolution, i.e. a small image size. The coding result is then referred to so as to code the image by a high resolution, i.e. a large image size. In coding a high resolution image, the data of the orthogonal transformation coefficients of the previously coded low resolution image is once inverted and brought back to a decoded image. This decoded image is then enlarged on the image space to become estimate data. Then with respect to each pixel of the high resolution image, difference data is obtained by making reference to the estimate data, after which the difference data is coded by the orthogonal transformation. In this system, the orthogonal transformation coefficients which are obtained by coding the low resolution image have to be once inverted, which means that an extra operation is required. Moreover, because an interpolation process using a band pass filter is required in enlarging the decoded image and generating the estimate data, there is a possibility that estimation errors should occur due to problems relating to operational precision and filtering characteristics of the interpolation process. In this respect, it becomes an issue that the coding efficiency is easily deteriorated.
Furthermore, as to another image size transformation method for enlarging or reducing a size of an image, there is known a method in which coefficients are added to or eliminated from in enlarging or reducing the image size. For example, according to an image enlargement method as disclosed in Japanese Patent Laid-Open Publication No. 2-76472, orthogonal transformation coefficients which are obtained by two dimentional discrete cosine transformation by a (Nxc3x97N) point block unit are applied to a low frequency side of a (Mxc3x97M) point block where M greater than N, while zero values are applied to a high frequency side of the block so as to obtain orthogonal transformation coefficients of (Mxc3x97M) points. Then by inverting the orthogonal transformation coefficients of (Mxc3x97M) points after transformation, by the two dimensional discrete cosine transformation of (Mxc3x97M) points, it is possible to obtain a decoded image being enlarged by M/N times. On the other hand, by eliminating the high frequency side of the orthogonal transformation coefficients of (Nxc3x97N) points, generating coefficients of a (Kxc3x97K) point block where K less than N, and inverting the coefficients by the two-dimentional discrete cosine transformation of (Kxc3x97K) points, it is possible to obtain a decoded image being reduced by K/N times.
FIG. 2 shows a concept of such enlarging and reducing process. As to a similar image size transformation method, there is known a high quality image enlargement method disclosed in Japanese Patent Laid-Open Publication No. 3-204268. In the disclosure, instead of carrying out the process of applying zero values to the high frequency side as in the former method, values being anticipated by an autoregressive model are applied from the coefficients of the low frequency side. By the application of such process, it is possible to obtain a seemingly enlarged decoded image with high resolution components being reinforced.
However, in respect with the image size transformation method of the two prior art disclosures mentioned above, various magnifications and reduction ratios such as (M/N) time enlargement and (K/N) time reduction are possible. On the other hand, in decoding the orthogonal transformation coefficients after having the image size transformed, inverse orthogonal transformation of different sample points must be carried out depending on the magnification or reduction ratio. Accordingly, in respect with a decoding means which is usually provided beforehand with only a predetermined inverse orthogonal transformation function, there is a possibility that the coded data cannot be decoded. Furthermore, in case when a decoding device applicable for various magnifications and reduction ratios is made possible by a hardware circuit, it might be a problem that the circuit structure would become complicated.
It is therefore an object of the present invention to provide a more efficient image size transformation method for an orthogonal transformation coded image, which is capable of enlarging and reducing a size of an image on orthogonal transformation coefficients, thus requiring a small operation amount.
According to a first aspect of the present invention there is provided an image size transformation method for an orthogonal transformation coded image, enlarging or reducing an image size of coded data, in respect with an image compressing and coding using an orthogonal transformation, comprising the steps of: reading out orthogonal transformation coefficients being the coded data; conducting a coefficient transformation by an operation process on the orthogonal transformation coefficients; and outputting the orthogonal transformation coefficients after having the image size enlarged or reduced, thus executing an enlargement or a reduction of the image.
According to a second aspect of the present invention, there is provided an image size transformation method for an orthogonal transformation coded image, enlarging an image size to output a decoded image, in respect with an image compressing and coding using an orthogonal transformation, comprising the steps of: reading out orthogonal transformation coefficients being coded by an N point orthogonal transformation, where N is a natural number; conducting a coefficient transformation by an operation process on the orthogonal transformation coefficients; calculating to obtain orthogonal transformation coefficients, after having the image size enlarged by a certain integer; and decoding the calculated transformation coefficients by an N point inverse orthogonal transformation so as to output an enlarged image, thus executing enlargement and decoding of a coded image.
According to a third aspect of the present invention, there is provided an image size transformation method for an orthogonal transformation coded image, coding an image into hierarchies to obtain a plurality of decoded images with different sizes, comprising a coding stage and a decoding stage. The coding stage includes the steps of: provided that an input image is being previously hierarchically organized according to different sizes; conducting an orthogonal transformation with respect to each hierarchy to obtain an orthogonal transformation coefficient; coding the orthogonal transformation coefficient of the lowest hierarchy with the smallest image size such that it should be outputted as coded data; obtaining difference data between an orthogonal transformation coefficient of a subject hierarchy and orthogonal transformation coefficients of the other lower hierarchies with smaller image sizes having been enlarged on the coefficients; and coding the obtained difference data such that it should be outputted as coded data. The decoding stage includes the steps of: conducting an inverse coding process on the coded data to obtain difference data between an orthogonal transformation coefficient of the lowest hierarchy and orthogonal transformation coefficients of the other hierarchies; adding together the difference data of the subject hierarchy and the orthogonal transformation coefficients of the other lower hierarchies except for the lowest having been enlarged by an enlargement process on the coefficients, so as to consecutively obtain through a calculation process an orthogonal transformation coefficient for each hierarchy starting from the lower hierarchies; and conducting an inverse orthogonal transformation on the obtained orthogonal transformation coefficient of each hierarchy to finally restore and output hierarchically decoded images.
According to a fourth aspect of the present invention, there is provided an image size transformation method for an orthogonal transformation coded image, reducing an image size to output a decoded image, in respect with an image compressing and coding using an orthogonal transformation, comprising the steps of: reading out orthogonal transformation coefficients being coded by an N point orthogonal transformation, where N is a natural number; conducting a coefficient transformation by an operation process on the orthogonal transformation coefficients; calculating to obtain orthogonal transformation coefficients, after having the image size reduced; and decoding the calculated transformation coefficients by an N point inverse orthogonal transformation so as to output a reduced image, thus obtaining a reduced decoded image of a coded image.
According to a fifth aspect of the present invention, there is provided an image size transformation method for an orthogonal transformation coded image, coding an image into hierarchies to obtain a plurality of decoded images with different sizes, comprising a coding stage and a decoding stage. The coding stage includes the steps of: conducting an orthogonal transformation on an input image to obtain orthogonal transformation coefficients; conducting a hierarchical reduction process on the obtained orthogonal transformation coefficients to obtain data of orthogonal transformation coefficients constructed in a plurality of hierarchies of different image sizes; coding the orthogonal transformation coefficient of the lowest hierarchy with the smallest image size such that it should be outputted as coded data; obtaining difference data between an orthogonal transformation coefficient of a subject hierarchy and orthogonal transformation coefficients of the other lower hierarchies with smaller image sizes having been enlarged on the coefficients; and coding the obtained difference data such that it should be outputted as coded data. The decoding stage includes the steps of: conducting an inverse coding process on the coded data to obtain difference data between an orthogonal transformation coefficient of the lowest hierarchy and orthogonal transformation coefficients of the other hierarchies; adding together the difference data of the subject hierarchy and the orthogonal transformation coefficients of the other lower hierarchies except for the lowest having been enlarged by an enlargement process on the coefficients, so as to obtain through a calculation process an orthogonal transformation coefficient for each hierarchy; and conducting an inverse orthogonal transformation on the obtained orthogonal transformation coefficient of each hierarchy to finally restore and output hierarchically decoded images.
With respect to the above fifth aspect of the present invention, in the decoding stage, the orthogonal transformation coefficient for each hierarchy can be obtained consecutively through a calculation process starting from the lower hierarchies.